Legionella Educational Series: Understanding Building Risk Factors

Posted by Stephen Betts on May 19, 2017 11:00:00 AM
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Legionella bacteria live in water and are found in natural aquatic environments.  However, the artificial environments within building systems can provide conditions that encourage the bacteria’s growth, such as ideal temperature, water stagnation, and presence of biofilm. Various water systems have been the source of legionellosis cases including potable water systems, cooling towers, spas, and other artificial systems. Understanding building risk factors is an important part of preventing Legionnaires' disease.  To cause harm, the bacteria must be present and grow in water, the colonized water needs to be transmitted to a person, and that person must be susceptible to infection. 

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Every facility is different in terms of water systems present, water temperature and quality, and ways occupants can be exposed to the bacteria.  Therefore, risk characterization is the first step towards developing a proactive approach to water safety.  While the only way to know if Legionella is present is to test for it, considering building risk factors is part of developing an effective risk management program.

Presence and Proliferation of Legionella in Building Water Systems

Building water systems provide conditions that can amplify Legionella bacteria, so it is important to consider how risk factors may apply to your facility.  According to the US Centers for Disease Control and Prevention (CDC), several factors can lead to Legionella growth within a building[1]

The following summarizes these key factors discussed by the CDC.

pH and Inadequate Disinfectant

Even if water entering a building includes a disinfectant such as chlorine, the water may still contain Legionella bacteria.  In addition to this, processes such as storage and heating reduce disinfectant levels, causing water to have minimal or no disinfectant, which may contribute to Legionella growth.  The pH of the water impacts disinfectant effectiveness, with highest effectiveness in a narrow range (~6.5 to 8.5).

Biofilm, Corrosion, Scale and Sediment

Microorganisms, including Legionella, colonize surfaces such as the interior of piping systems, resulting in a “biofilm”.  This film protects Legionella and provides food that supports bacterial growth.  Scale and corrosion can increase biofilm formation and protect microorganisms from disinfectants.

Water Pressure Changes

Changes in water pressure can dislodge biofilm, potentially colonizing the piping network downstream.

Water Stagnation

Stagnation of water or areas with low flow can promote biofilm growth, reduce water temperatures, and reduce the level of disinfectant in the water.  This can occur in piping or storage tanks that are infrequently used, within dead legs, or at irregularly used fixtures.

Water Temperature Fluctuations

Legionella bacteria have been shown to grow best at temperatures between 25°C and 42°C, although growth can occur outside of this range.  These temperatures are commonly found in building water systems due to settings on hot water heaters and mixing valves, heat loss in distribution piping, stagnation, and other factors.  ASHRAE Guideline 12[2] recommends that where practical, hot water should be stored above 60°C with a return temperature of at least 51°C.


Potable water systems, cooling towers, decorative water features, whirlpools, and other systems generate water in an aerosolized form that may contain Legionella bacteria.  Transmission to people occurs by inhalation of this airborne water.  In addition, building occupants may aspirate water directly into the lungs, a process often referred to as water “going down the wrong pipe”.  Identifying these pathways of potential exposure is an important part of characterizing risk in a building.


Anyone can be affected by Legionella bacteria, but certain people are known to be at higher risk.  According to the ASHRAE 188 Standard, this includes “the elderly, dialysis patients, persons who smoke, and persons with medical conditions that weaken the immune system”[3].  When characterizing risk in a building, it is important to consider which building occupants are served by which water systems, and how people might be exposed.

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Risk Factor Examples

Using a couple of examples, let’s consider how risk factors could apply to the design and operation of a building. 

Potable hot water systems are a source of aerosolized water, in particular from faucets and showers.  There may be areas of stagnation and low flow.  The ability to achieve ASHRAE best practices with respect to temperature control – storage above 60°C with a return temperature of at least 51°C – will depend on various characteristics of your facility, such as building codes, hot water system configuration, use of mixing valves, and protection from scald risk.  Finally, we should think about susceptibility.  Where are the most at-risk patients, and how are they exposed?

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Cooling towers provide another example.  Appropriate water treatment is crucial – these systems should have a water treatment program designed to control microorganisms – but other factors can come into play as well.  For example, does operation of the building lead to periods of stagnant water, such as during shut-downs?  Is there a source of nutrients nearby that might promote bacterial growth?  Have there been recent construction or water pressure changes?  These factors can influence the potential for Legionella to amplify and be transmitted to susceptible building occupants.



Carefully considering your building’s risk factors is the first step towards managing water safety.  In fact, this is now an industry best practice.  A hazard assessment is part of compliance with the ASHRAE 188 Standard, released in June 2015.  This standard requires certain facilities (including healthcare facilities) to have a risk management program, also called a water management program or Water Safety Plan, for potable and specific non-potable water systems. 

There is no one-size-fits-all solution with respect to managing risk in building water systems.  The water management program should be informed by an assessment of risk, considering building risk factors and Legionella sampling as appropriate.  It becomes a road map for decision making and implementation of further risk reduction measures if required.  Some of these measures may include secondary disinfection (e.g., monochloramine, copper-silver, chlorine dioxide), temperature changes, or point of use filtration.  However, none of these measures should be implemented unless justified by risk characterization and alignment with the Water Safety Plan.

Your water management vendor should be able to guide you through the risk assessment and Water Safety Plan development process.


Interested in learning more about Water Safety? 

Read Six Steps To Develop A Water Safety Plan And Comply With ASHRAE Standard 188-2015 – Legionella

Read Point-of-Use Filters for Control of Microorganisms in Building Water Systems: 5 Things You Should Know


[1] US Centers for Disease Control and Prevention (2016). “Developing a Water Management Program to Reduce Legionella Growth & Spread in Buildings”.

[2] ASHRAE (2000). “ASHRAE Guideline 12: Minimizing the Risk of Legionellosis Associated with Building Water Systems”

[3] ASHRAE (2015) Standard 188: Legionellosis: Risk Management for Building Water Systems.

Topics: Water Safety

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